This document is as it is taken from Udemy course.
Imperative Commands
While you would be working mostly the declarative way - using definition files, imperative commands can help in getting one-time tasks done quickly, as well as generate a definition template easily. This would help save a considerable amount of time during your exams.
Before we begin, familiarize yourself with the two options that can come in handy while working with the below commands:
--dry-run: By default, as soon as the command is run, the resource will be created. If you simply want to test your command, use the --dry-run=client option. This will not create the resource. Instead, tell you whether the resource can be created and if your command is right.
-o yaml: This will output the resource definition in YAML format on the screen.
Use the above two in combination along with Linux output redirection to generate a resource definition file quickly, that you can then modify and create resources as required, instead of creating the files from scratch.
kubectl run nginx --image=nginx --dry-run=client -o yaml > nginx-pod.yaml
POD
Create an NGINX Pod
kubectl run nginx --image=nginx
Generate POD Manifest YAML file (-o yaml). Don’t create it(—dry-run)
kubectl run nginx --image=nginx --dry-run=client -o yaml
Deployment
Create a deployment
kubectl create deployment --image=nginx nginx
Generate Deployment YAML file (-o yaml). Don’t create it(—dry-run)
kubectl create deployment --image=nginx nginx --dry-run -o yaml
Generate Deployment with 4 Replicas
kubectl create deployment nginx --image=nginx --replicas=4
You can also scale deployment using the kubectl scale command.
kubectl scale deployment nginx --replicas=4
Another way to do this is to save the YAML definition to a file and modify
kubectl create deployment nginx —image=nginx--dry-run=client -o yaml > nginx-deployment.yaml
You can then update the YAML file with the replicas or any other field before creating the deployment.
Service
Create a Service named redis-service of type ClusterIP to expose pod redis on port 6379
kubectl expose pod redis --port=6379 --name redis-service --dry-run=client -o yaml
(This will automatically use the pod’s labels as selectors)
Or
kubectl create service clusterip redis --tcp=6379:6379 --dry-run=client -o yaml (This will not use the pods’ labels as selectors; instead it will assume selectors as app=redis. You cannot pass in selectors as an option. So it does not work well if your pod has a different label set. So generate the file and modify the selectors before creating the service)
Create a Service named nginx of type NodePort to expose pod nginx’s port 80 on port 30080 on the nodes:
kubectl expose pod nginx --port=80 --name nginx-service --type=NodePort --dry-run=client -o yaml
(This will automatically use the pod’s labels as selectors, but you cannot specify the node port. You have to generate a definition file and then add the node port in manually before creating the service with the pod.)
Or
kubectl create service nodeport nginx --tcp=80:80 --node-port=30080 --dry-run=client -o yaml
(This will not use the pods’ labels as selectors)
Both the above commands have their own challenges. While one of it cannot accept a selector the other cannot accept a node port. I would recommend going with the kubectl expose command. If you need to specify a node port, generate a definition file using the same command and manually input the nodeport before creating the service.
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